*{ margin:0px; padding:0px;}
body 
{
    line-height:1.5;
    font-family:微软雅黑;
}
a{ text-decoration:none;}
ul,li{ list-style:none; }


section 
{
    min-height:100vh;   
    position: relative; 
    display: flex ;  
    align-items: center; 
    background:#000000;
    color:#ffffff;
    justify-content: center;
}

section p{ font-size:40px; }
.main-nav {
    position: fixed;
    width: 100%;
    padding-top: 15px;
    padding-bottom: 15px;
    z-index: 1;
    background:#3e3e3e; 
    display: flex ;  
    justify-content: flex-end;
}
.main-nav ul{ 
    display: flex ;  
}
.main-nav ul li a 
{
    color:#ffffff; 
    padding:0px 20px;     
    font-size: 12px;
    letter-spacing: 1px;
        position: relative;
}
.main-nav.bg-dark {
    background-color: #726666 ;
}

.main-nav a:after {
    content: ' ';
    position: absolute;
    bottom: -10px;
    left: 50%;
    transform: translateX(-50%);
    -moz-transform: translateX(-50%); 
    -o-transform: translateX(-50%);
    -ms-transform: translateX(-50%);
    height: 2px;
    width: 0;
    background-color: #fff;
    transition: all .5s;
    -webkit-transition: all .5s;
    -o-transition: all .5s;
    -ms-transition: all .5s;
}
.main-nav li.active a:after {
    width: 20px;
}